Wireless energy (or power) transfer is a promising approach for environmentally friendly, convenient and reliable powering of electrical and electronic devices, such as computers, electric vehicles, cell phones, etc.
Resonant Inductive Coupling pioneered by Nikola Tesla in the early 20th century has later found applications in power transfer systems.
Recent developments in the field of power transfer have demonstrated the ability to transfer 60 W power with 40% efficiency covering 2 m distance. This medium-range wireless energy transfer system (called “WiTricity”) has been developed by a group of MIT scientists based on strong coupling between electromagnetic resonant objects, i.e., transmitters and receivers that contain magnetic loop antennas critically attuned to the same frequency. As presented in A. Karalis, et al., “Efficient Wireless Non-Radiative Mid-Range Energy Transfer”, Ann. Phys., 10.1016 (2007), and U.S. Pat. Nos. 7,741,734 and 7,825,543, the system for wireless energy transfer includes a first resonator structure configured to transfer energy non-radiatively to a second resonator structure over medium range distances. These distances are characterized as being large in comparison to transmit-receive antennas, but small in comparison to the wavelength of the transmitted power.
The resonators in these energy transfer systems are formed as self-resonant conducting coils from a conductive wire which is wound into a helical coil of a predetermined radius r and height h surrounded by air, as shown in FIG. 1. The non-radiative energy transfer in this system is mediated by a coupling of a resonant field evanescent tail of the first resonator structure and a resonant field evanescent tail of the second resonator structure.
The ability to effectively transfer power over desired distances, depends on losses in the resonance system which may be attributed to ohmic (material absorption) loss inside the wire, radiative loss in the free space, as well as dielectric losses in dielectric materials used in the system.
In “WiTricity,” the maximum power coupling efficiency of coils fabricated from standard conductors occurs at the 10 MHz range, where the combination of resistive and radiative losses are at a minimum. The effective range of these systems, i.e., a few meters at non-negligible efficiencies, has significant application within everyday life to provide power to personal electronics (laptops, cell phones) or other equipment within the confines of room. However, this type of system is incapable of efficient power transfer with respect to relatively long range applications.
It would be highly desirable to extend the reach of the resonant inductive power transfer for applications in space, for example, for the on-orbit power transfer between the elements of a satellite cluster or on the surface of the moon between a centralized power station and a rover. In order to attain greater distances in wireless power transfer, higher efficiencies of power transfer are necessary. Therefore, it is highly desirable to provide a long range power transfer system where the loss paths existing in the mid-range system are minimized or eliminated.